Directional radiation and photothermal effect enhanced control of 2D excitonic emission based on germanium nanoparticles.

Dielectric nanostructures with Mie resonances have shown promising applications for building nanoantennas and metasurfaces. Coupling between Mie resonators and transition metal dichalcogenide (TMDC) monolayers is of great significance, because the existence of Mie resonances can modulate phases and radiation directions effectively. Recently, monolayer binary and ternary TDMCs have drawn more attention owing to the intriguing and tunable excitonic states from visible to near infrared. However, the coupling mechanism between monolayer TMDCs and Mie resonators has not been well studied. Moreover, it is still a great challenge to realize the control of excitonic emission wavelength and intensity simultaneously. Here, for the first time, we demonstrate that germanium nanoparticles (Ge NPs), a typical high refractive index dielectric Mie resonator, are capable of controlling both the intensity and direction of PL emissions in the near-infrared from monolayer WSe2(1-x)Te2x. Through putting Ge NPs below or above monolayers, we observed the obvious emission directivity because of the higher refractive index and higher loss of Ge than silicon. Besides, higher absorption in Ge NPs brings photothermal effects during the interaction with TMDCs. These findings indicate that Ge-based Mie resonators may guide the design of new type nanophotonics devices in the future.

Direct-indirect bandgap transition in monolayer MoS2 induced by an individual Si nanoparticle.

MoS2 is promising for the next generation of electronic and optoelectronic devices by virtue of its unique optical, electrical and mechanical properties. Bandgap engineering of it is an interesting topic. However, the reported factors including temperature, defect, strain and external electric field are difficult to handle precisely. Here, we demonstrated direct-indirect bandgap transition in monolayer MoS2 induced by an individual Si nanoparticle. We observed photoluminescence (PL) emission with obvious spectral redshift and broadening in the MoS2/Si heterostructures after depositing Si nanoparticles onto the surface of monolayer MoS2. Raman spectra of heterostructures show measurable shifts in contrast with the bare MoS2. Energy transfer between MoS2 and Si nanoparticles did not happen, which is demonstrated by scattering spectra of MoS2/Si heterostructures. In addition, the natural oxide layer presented on the surface of Si nanoparticles can effectively prevent the carrier transferring from Si nanoparticles to MoS2. Thus, we attribute the direct-indirect bandgap transition of monolayer MoS2 to the strain induced by Si nanoparticles controlled by their sizes. The PL intensity of MoS2/Si heterostructure depends on the size of Si nanoparticles, resulting from the enhanced optical absorption of monolayer MoS2 based on Mie resonances of Si nanoparticles. The MoS2/Si heterostructure is promising for photodetector and circuit integration.

Trapping and filtering of light by single Si nanospheres in a GaAs nanocavity.

The arbitrary manipulation of optical waves in the subwavelength dimension is a fundamental issue for the microminiaturization and integration of optic parts. In the past decade, major efforts were focused on the surface plasmon resonance mostly exhibited by metallic nanostructures, which could effectively capture and concentrate the visible light at the cost of high levels of intrinsic losses. However, the use of all-dielectric nanostructures can avoid the abovementioned problem due to lower intrinsic losses and the presence of abundant resonance modes. Herein, as a kind of building block for light manipulation, GaAs nanogrooves were fabricated and studied to obtain comprehensive information about the resonance modes in an individual all-dielectric nanogroove; by placing a single Si nanosphere in an isolated GaAs nanocavity, the nanogroove scattering could be controlled depending on the coupling strength of nanogrooves. The Lorentzian line approximation and harmonic oscillator coupling model were used to pursue the interactions among the resonance modes. Experimental and theoretical studies showed that this heterostructure could trap the broadband visible light in the back and filter the light with a specific wavelength in the front. These findings suggest that the proposed heterostructure can act as a light filter and an antenna on nanophotonic chips due to its unique optical properties.

Tunable Control of Interlayer Excitons in WS2/MoS2 Heterostructures via Strong Coupling with Enhanced Mie Resonances.

Strong Coulomb interactions in monolayer transition metal dichalcogenides (TMDs) produce strongly bound excitons, trions, and biexcitons. The existence of multiexcitonic states has drawn tremendous attention because of its promising applications in quantum information. Combining different monolayer TMDs into van der Waals (vdW) heterostructures opens up opportunities to engineer exciton devices and bring new phenomena. Spatially separated electrons and holes in different layers produce interlayer excitons. Although much progress has been made on excitons in single layers, how interlayer excitons contribute the photoluminescence emission and how to tailor the interlayer exciton emission have not been well understood. Here, room temperature strong coupling between interlayer excitons in the WS2/MoS2 vdW heterostructure and cavity-enhanced Mie resonances in individual silicon nanoparticles (Si NPs) are demonstrated. The heterostructures are inserted into a Si film-Si NP all-dielectric platform to realize effective energy exchanges and Rabi oscillations. Besides mode splitting in scattering, tunable interlayer excitonic emission is also observed. The results make it possible to design TMDs heterostructures with various excitonic states for future photonics devices.

Directional Fano Resonance in an Individual GaAs Nanospheroid.

Fano resonance has been observed in a wide variety of nanophotonic structures such as photonic crystals, plasmonic structures, and metamaterials. It arises from the interference of discrete resonance states with broadband continuum states. As an emerging nanophotonic material, high-index all-dielectric nanomaterials provide a new platform to achieve Fano resonance by virtue of the simultaneous excited electric and magnetic resonances. However, to date, Fano resonance in the visible region has not been observed in individual high-index all-dielectric nanoparticles. Here, for the first time, the experimental observation of the directional Fano resonance is reported in an individual GaAs nanospheroid. The special geometry enables GaAs nanospheroids to generate spectrally overlapped electric and magnetic dipole resonances, which enhances their spectral coupling, giving rise to asymmetric-shaped backward scattering spectrum. This directional Fano resonance can be tuned by the aspect ratio of nanospheroids as well as excitation polarization. In addition, efficient directional light scattering is realized at the total scattering peak of the GaAs nanospheroid. The forward-to-backward scattering ratio can be largely enhanced due to Fano dip in the backward scattering spectrum. These findings suggest that high-index all-dielectric nanospheroid is a promising candidate for directional sources and optical switches.

Active tuning of the Fano resonance from a Si nanosphere dimer by the substrate effect.

All-dielectric materials have aroused great interest for their unique light scattering and lower losses compared with plasmonics. Generally, optical properties made by all-dielectric materials can be passively controlled by varying the geometry, size and refractive index at the design stage. Therefore, the realization of active tuning in the field of nanophotonics is important to improve the practicality and achieve light-on-chip technology in the future. Herein, we combine the high refractive index of Si and the phase transition of VO2 to form an active tuning hybrid nanostructure with higher quality factor by depositing Si nanospheres on the VO2 layer with an Al2O3 substrate. As the temperature goes up, the refractive index of the VO2 layer switches from high to low. The scattering intensity of the magnetic dipole resonance of Si nanospheres decreases differently depending on their size, while the intensity of the electric dipole resonance remains almost unchanged. Meanwhile, Fano resonances are observed in the Si nanosphere dimers with a continuous variable Fano lineshape when adjusting the temperature. Mie theory and substrate-induced resonant magneto-electric effects are used to analyze and explain these phenomena. Tuning of the Fano resonance is attributed to the substrate effect from the interaction between Si nanospheres and phase transition of the VO2 layer with temperature. These light scattering properties of such a hybrid nanostructure make it promising for temperature sensing or as a light source at the nanometer scale.

The optical duality of tellurium nanoparticles for broadband solar energy harvesting and efficient photothermal conversion.

Nanophotonic materials for solar energy harvesting and photothermal conversion are urgently needed to alleviate the global energy crisis. We demonstrate that a broadband absorber made of tellurium (Te) nanoparticles with a wide size distribution can absorb more than 85% solar radiation in the entire spectrum. Temperature of the absorber irradiated by sunlight can increase from 29° to 85°C within 100 s. By dispersing Te nanoparticles into water, the water evaporation rate is improved by three times under solar radiation of 78.9 mW/cm2. This photothermal conversion surpasses that of plasmonic or all-dielectric nanoparticles reported before. We also establish that the unique permittivity of Te is responsible for the high performance. The real part of permittivity experiences a transition from negative to positive in the ultraviolet-visible-near-infrared region, which endows Te nanoparticles with the plasmonic-like and all-dielectric duality. The total absorption covers the entire spectrum of solar radiation due to the enhancement by both plasmonic-like and Mie-type resonances. It is the first reported material that simultaneously has plasmonic-like and all-dielectric properties in the solar radiation region. These findings suggest that the Te nanoparticle can be expected to be an advanced photothermal conversion material for solar-enabled water evaporation.

Application of composite dissolving microneedles with high drug loading ratio for rapid local anesthesia.

We report a type of dissolving microneedles (DMNs) which was made of composite matrix materials of hydroxypropyl methyl cellulose (HPMC) and poly(methylvinylether­co­maleic anhydride) (PMVE/MA copolymer, Gantrez S-97), and was successfully loaded with lidocaine hydrochloride. The weight of lidocaine hydrochloride loaded in the microneedles tip was 70% of the weight of the whole tip. The content was 3.43 ±â€¯0.12 mg in weight, which was determined by high performance liquid chromatography (HPLC). The results for mechanical test showed that these microneedles were able to penetrate into the skin of the experimental animals, that was proved using organic staining, texture analyzer and histological examination. The fracture force of the microneedles was 5.442 ±â€¯0.412 N, which was much higher than the one required for the skin penetration. The DMNs with lidocaine hydrochloride could be dissolved inside of the rat skin in 5 min. The onset time would be faster (in <5 min) when it was applied to the guinea pig model, in comparing with a commercially available anesthesia cream that had an onset time for 100 min. However, the efficacy of the DMNs for the local anesthesia only lasted for 16 min. It was shorter than that of the commercially available anesthesia cream with which the efficacy could last for about 130 min. After the DMNs was packaged under the vacuum and dark condition, it was stable for 3 months under the condition of a temperature of 40 ±â€¯2 °C and a humidity of 75 ±â€¯5%. The result of the experiment for the safety evaluation showed that the microneedles were non-irritating and non-allergenic to the skin. In conclusion, the DMNs with lidocaine hydrochloride could be safely administered to the skin with a quick onset time for the local anesthesia.